1. Field of the Invention
The present invention relates to an email relay apparatus and an email relay method for relaying an email attached with a moving image to a destination. The present invention also relates to a program causing a computer to execute the email relay method.
2. Description of the Related Art
An image obtained by a digital camera, a scanner, or the like can represent 24-bit colors (that is, approximately 16.77 million colors) wherein 8 bits each are assigned to R, G, and B. However, some reproduction apparatus are capable of reproducing fewer colors such as 256 colors. Furthermore, an image format used frequently in Web sites (such as GIF or PNG) can deal with only 256 colors. Therefore, for displaying an image obtained by a digital camera or the like on a reproduction apparatus which can display fewer colors, or for converting the image into an image format such as GIF or PNG, the 16.77 million colors need to be reduced to 256 colors.
In conventional color reduction processing, palette data comprising 256 palette colors are generated for optimally representing the image to be displayed, and one of the 256 palette colors is selected from the palette data for color conversion on each pixel. In this manner, the colors in the image are reduced to the 256 colors.
Moreover, a method of generating a moving image has been proposed in Japanese Unexamined Patent Publication No. 11(1999)-259640. In this method, a moving image obtained by a digital camcorder is subjected to real-time color reduction processing, and an image having motion caused by serially changing still images (hereinafter referred to as a reduced-color moving image) is generated. In this method, frames that change at every 1/60 of a second in the moving image are subjected to the real-time color reduction processing without being skipped. Therefore, palette data are generated for every several frames instead of each frame, and the color reduction processing is carried out by using the palette data for the several frames.
Meanwhile, functions of mobile terminals such as camera-embedded mobile phones have improved remarkably, and a communication speed is also rising. Therefore, a mobile terminal has been realized that is capable of photographing a moving image, transmitting the moving image by attaching the image to an email, and reproducing desired moving image content by downloading the moving image content through access to a moving image transmission server.
In addition, an email relay server has also been proposed that can carry out appropriate image processing in accordance with the type of a terminal as a receiver of an email attached with a still image sent from a camera-embedded mobile phone (Japanese Patent Application No. 2002-214215). By using such an email relay server, the image processing can be carried out on image data to cause the image to be appropriate for a size, brightness, and colors of a display screen of the terminal, and the image data are sent to the terminal by being attached to the email.
Another system has also been proposed for sending image data (Japanese Unexamined Patent Publication No. 2002-244971). In this system, image data in accordance with models of mobile terminals are stored in an image server corresponding to a Web site, and the model of a mobile terminal that accesses the Web site is judged. The image data corresponding to the judged model are then sent to the mobile terminal.
Meanwhile, following a recent increase in the number of pixels in a display screen of a mobile terminal, a mobile terminal having a display screen that is 2×2 (in other words, 4) times larger in the number of pixels in horizontal and vertical directions is on the market. If an image appropriate for the number of pixels in such a display screen of a mobile terminal is used, the image can be displayed on the mobile terminal in higher definition.
Furthermore, in the case where an email attached with a moving image is sent to a mobile terminal, the moving image can surely be displayed thereon if the terminal is capable of moving image display. However, there are some models of mobile terminals that cannot display a moving image. Therefore, even if an email attached with a moving image having been subjected to appropriate image processing in accordance with the model of a mobile terminal as a receiver is sent to the mobile terminal, the moving image may not be displayed thereon. In this case, the moving image can be converted into a reduced-color moving image as has been described above, and sent to the mobile terminal as the receiver.
Meanwhile, regardless of an increase in the number of pixels in a display screen, capacity of a memory installed in a mobile terminal for image display is not large due to high cost or the like. For example, although the number of pixels in a display screen has achieved a fourfold increase, memory capacity has only been doubled.
In addition, a format of a reduced-color moving image such as GIF and PNG uses a compression method considering one-dimensional redundancy (that is, redundancy in only one direction of an image). Therefore, although a compression ratio can be improved in the case where pixels having the same value are aligned in one direction, the compression ratio cannot be increased for a direction orthogonal thereto.
For this reason, in the case where a reduced-color moving image of a reduced-color moving image format is sent and displayed on a display screen of a mobile terminal having a large number of pixels (hereinafter referred to as a high-resolution display screen), a data size of the image after compression can only be doubled although the number of pixels therein can be quadrupled, for example. Therefore, the data size of the image in the reduced-color moving image format needs to be decreased by degrading image quality, decreasing frames in the image to be displayed, or lowering resolution, for example. However, since the image of the reduced-color moving image format originally has only 256 displayable colors, quality of the image displayed on the mobile terminal is substantially degraded by the data size reduction despite that the mobile terminal can carry out high-definition image display.